1. Field of the Invention
This invention relates to a vehicle slip control system which controls the driving force according to slip condition of the driving wheels.
2. Description of the Prior Art
There has been known a traction control system for a vehicle which suppresses driving force of the driving wheels in order to prevent the driving wheels from slipping due to an excessive driving torque to deteriorate starting performance and/or acceleration performance. Such a traction control is initiated, for instance, when the difference between the wheel speed of the driving wheels and the driven wheels detected by wheel speed sensors exceeds a predetermined value, and the engine output and/or the braking force applied to the driving wheels are feedback-controlled so that the difference in the wheel speeds converges on a predetermined target value.
On a slippery road such as that covered with snow, the vehicle can skid due to difference in the friction coefficient between the parts of the road in contact with the left and right driving wheels when an excessive driving torque is transmitted to the driving wheels. Such a phenomenon is especially remarkable in a vehicle provided with a driving force distributing means such as a differential in a driving force transmission path between the engine and the left and right driving wheels.
In order to overcome the problem, as disclosed, for instance, in Japanese Unexamined Patent Publication No. 60(1985)-128028, there has been known a traction control system in which the wheel speeds of the left and right driving wheels are separately detected and the driving torque output from the power plant is reduced while there is a difference between the wheel speeds of the left and right driving wheels. With this arrangement, the engine output is reduced so long as there is a difference between the wheel speeds of the left and right driving wheels, whereby excessive driving force cannot be transmitted to the driving wheels.
However, if the driving torque is always reduced when there is a difference between the wheel speeds of the left and right driving wheels as in the traction control system described above, there arises the following problem when running a so-called split surface road where the friction coefficient of the part of the road in contact with the left driving wheel differs from that in contact with the right driving wheel. That is, the driving torque is reduced so long as the road gripping force of one of the driving wheels is somewhat poor even if one of the driving wheels can grip the road surface in such an extent that ensures normal running and accordingly, the starting performance and the accelerating performance are suppressed in spite of the driver's will.
Thus it has been proposed to shift the control, when running a split surface road, from a normal control where the driving force is simply suppressed when the driving wheels are spinning to a split control where the engine output is controlled attaching importance to the starting performance and the accelerating performance. However in order to realize such a traction control system, the split surface road must precisely detected. For example, the left and right driving wheels can alternately spin on a low friction coefficient road such as a frozen road, and in such a case, there is generated a difference between the wheel speeds of the left and right driving wheels, which can lead to the mistake of determining that the vehicle is running a split surface road. Further, spin of the slipping driving wheel can temporarily subside during running on split surface road and when the split control is simply interrupted in such a case, the response of the system upon resumption of spin deteriorates.